Grooved impactor and inertial trap for sampling inhalable particulate matter

ABSTRACT

An inertial trap and grooved impactor for providing a sharp cutoff for particles over 15 microns from entering an inhalable particulate sampler. The impactor head has a tapered surface and is provided with V-shaped grooves. The tapered surface functions for reducing particle blow-off or reentrainment while the grooves prevent particle bounce. Water droplets and any resuspended material over the 15 micron size are collected by the inertial trap and deposited in a reservoir associated with the impactor.

BACKGROUND OF THE INVENTION

The invention described herein arose at the Lawrence Berkeley Laboratoryunder Contract No. W-7405-ENG-48 between the United States Department ofEnergy and the University of California.

The invention relates to air pollution monitoring, particularly toapparatus for collecting air samples, and, more particularly, to aninertial trap and a grooved impactor for sampling inhalable particulatematter.

In many cases where air pollution is monitored, it is desired to collectonly the pollutants likely to cause harm to people. Since peoplenormally have a built-in filter system that prevents airborne particleslarger than 15 micrometers from reaching the respiratory system, it isnecessary to remove all such larger particles if an accurate sampling ofpotentially harmful particulate matter is to be collected.

Particulate samplers of various types are known in the art asexamplified by U.S. Pat. No. 3,252,323, issued May 24, 1966, to W. L.Torgeson; U.S. Pat. No. 3,795,135, issued Mar. 5, 1974, to A. A.Anderson; U.S. Pat. No. 4,133,202, issued Jan. 9, 1979, to V. A. Marple;and U.S. Pat. No. 4,255,172, issued Mar. 10, 1981, to M. L. Smith. Inaddition, co-pending U.S. Application Ser. No. 134,351 filed Mar. 27,1980, now U.S. Pat. No. 4,301,022 issued Nov. 17, 1981, describes andclaims a high efficiency virtual impactor for dividing aparticle-containing gas flow into coarse and fine particle-containingflows for particle collection. Further, an inertial impactor utilizing asimple cup impactor is described in an article by B. Y. H. Liu et alentitled "Aerosol Sampling Inlets and Inhalable Particles", ParticleTechnology Laboratory Publication No. 397, University of Minnesota,Minneapolis, Minn., October 1979.

Problems associated with the prior known particle sampling apparatusrelate to particle bounce, reentrainment, and the accumulation of debrisor water which may affect the critical geometry in the impaction region,such that particles larger than those desired (15 m) are not reentrainedin the smaller particle flow. Thus, a need recognized in the art, is amethod or apparatus for reducing or eliminating particle bounce andassociated reentrainment thereof. Various prior art apparatus have beendirected to separating material by use of curved members, ridged orangled members, etc., which function to slow the flow of the materialduring separation thereof. Such is exemplified by U.S. Pat. No. 555,553,issued Mar. 3, 1896, to E. Austin; U.S. Pat. No. 1,519,428, issued Dec.16, 1924, to J. A. Wilisch; U.S. Pat. No. 3,623,828, issued Nov. 30,1971, to H. Shapiro; and U.S. Pat. No. 4,275,566, issued June 30,, 1981,to J. W. Bonn. While the prior known apparatus have been effective inproducing the desired results, none has provided a solution to theabove-described problem associated with particle bounce, reentrainment,etc., in air monitoring and sampling apparatus.

Therefore, it is an object of this invention to provide a particulatesampling apparatus which substantially eliminates the problemsassociated with particle bounce and reentrainment.

A further object of the invention is to provide a particulate samplingdevice which includes an inertial trap and impactor.

Another object of the invention is to provide an apparatus for samplingairborne particles which will provide a sharp transmission efficiencyparticle cutoff at 15 micrometers (or other particle size cut desired).

Another object of the invention is to provide an inertial trap andimpactor for inhalable particulate matter which utilizes a tapered andgrooved impactor head for preventing particle bounce and particlereentrainment.

Other objects of the invention will become apparent to these skilled inthe art in view of the following description and accompanying drawings.

SUMMARY OF THE INVENTION

The present invention provides a solution to the above-referencedproblems associated with particle bounce, reentrainment, etc., in aparticulate-sampling apparatus. This is accomplished by utilizing asampling apparatus having an inertial trap and impactor which providesthe desired sharp cutoff for particles over 15 microns, wherein theimpactor head has a tapered upper surface provided with V-shapedgrooves. The grooves prevent or substantially eliminate particle bounce,and any particle blow-off or reentrainment is intercepted by theinertial trap and directed into a partitioned reservoir.

More particularly, the present invention provides an inhalablepaticulate sampler with a 15-micron cutoff and a flow rate of about 1 m³/hr. The particle bounce by a single backscattering of very elasticspherical particles is eliminated by covering the impactor head withconcentric V-shaped grooves with opening angles of less than 90°.Backscattering through double and tripple bounces are also suppressed ifthe opening angles of the grooves are designed to be under 45° and 30°respectively. A practical 15 μm cutpoint design calls for grooves about1.27 mm wide with an opening angle of 35° and forming concentric ringson a 5° taper or slope on the impactor head from the center outwardly.The impactor is fully shielded from the external wind by a coverarrangement. The downward slope of the impactor head helps to direct anywater and loose material into the peripheral inertial trap andeventually into a reservoir therebelow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cut-away view of an embodiment of a portion of an inhalableparticulate sampler utilizing the inertial trap and impactor of theinvention;

FIG. 2 illustrates an enlarged cross section of the particle sizeseparator of the FIG. 1 sampler mechanism; and

FIG. 3 illustrates a further enlarged section of the impactor andinertial trap of FIG. 2, made in accordance with the invention.

DETAILED DESCRIPTION OF THE INVENTION

The invention is directed to an inertial trap and impactor for inhalableparticulate inlets of a particle size separator having a particulateinlet with a 15-micron cutoff and capable of operation with a flow rateof 1 m³ /hr, for example. A compact impactor cup is used to achieve asharp cutoff with minimal wall loss and disturbance due to the localwind trajectories. An inertial trap and a partitioned reservoir are usedto arrest particle bounce, reentrainment and collect water, etc. Theworst case of particle bounce by backscattering of very elasticspherical particles is limited to under 2% by covering the impactor headwith concentric V-shaped grooves with opening angles of about 35°. Theimpactor head is provided with a taper or downward slope of about 5°,for example, which helps to direct any water and loose material into theperipheral inertial trap and eventually into a reservoir having a volumeof 60 ml, for example.

Referring now to the drawings, FIG. 1 shows a cut-away of an upperportion of an inhalable paticulate sampler utilizing a particle-sizeseparator, shown in greater detail in FIG. 2, which incorporates theinertial trap and impactor arrangement of the present invention, shownin detail in FIG. 3.

The illustrated portion of the inhalable particulate sampler of FIG. 1basically comprises an inlet section generally indicated at 10, and aparticle-size separator section generally indicated at 11 which isadapted to be attached via connection 12 to detection and monitoringmechanism, not shown. The inlet section 10 basically consists of afunnel-shaped assembly 13 terminating in an acceleration nozzle 14, anda cover assembly 15 is positioned on and secured to funnel-shapedassembly 13 through which air to be monitored passes into particle-sizeseparator section 11. The air enters through an annular slit between adeflection cone 16 and a guard-ring 17 of the cover assembly 15 whichhelps to make the inlet section 10 more weatherproof by stoppingrunovers from rain splashes. Variations in efficiency as a function ofwind speed have been reduced by deflecting the incoming air stream bythe deflection cone into a deceleration chamber 18 formed byfunnel-shaped assembly 13. The stagnated air is then passed throughnozzle 14 onto an impactor 19 of the particle-size separator section 11.

FIG. 2 illustrates the particle-size separator section 11 and the airflow therethrough. The air from inlet section 10 is drawn throughacceleration nozzle 14 and directed onto an upper surface 20 of a head21 of impactor 19, head 21 having a slightly conical configurationproduced by a downward or outward slope of 5°, for example, asillustrated in FIG. 3. The downward slope of the upper surface ofimpactor head 21 may vary from 2° to 8° and helps to direct any waterand loose material into a peripheral inertial trap 22 formed by a spaceitermediate impactor head 21 and a wall-forming member 23 positionedwithin an outer tubing or housing 24. Material passing into trap 22 iscollected in a reservoir 25 which may have a volume of 60 ml, forexample. Any angular momentum in the incoming air jet or asymmetry inthe radial flow at the impactor surface 20 tends to cause somestreamlines to make a spiral excursion into the reservoir 25 and depositparticles unintentionally. Such penetration of streamlines into thereservoir reduces the sharpness of the impactor transmission curve. Tominimize this undesired effect, four (4) partition vanes 26 (only twoshown in FIG. 1) are secured in reservoir 25 between impactor 19 andwall-forming member 23 to divide reservoir 25 into four (4) sections.Wall-forming member 23 includes an upper section 27 of greater thicknesswhich is located with respect to impactor head 21 so as to define thedesired entrance distance or mouth width of inertial trap 22. The foldedair flow geometry in the particle size separator section 11, asindicated by the flow arrows in FIG. 2, gives rise to a compact externalbody which minimizes the interference by the inlet section 10 on theoncoming wind trajectories.

An important objective of the particle-size separator section 11 is theability to perform reliable, unattended sampling in the field for anextended period. For example, many sampling devices use impactors whichemploy various techniques for coating the impactor surface with a stickysubstance to prevent particle bounce. Particle bounce at the impactorhead can be classified into two categories depending on whether the exitangle of the particle with respect to the streamlines near the surfaceis large or small. As can be seen in FIG. 3, if a large particle doesnot adhere to the impactor head it tends to be swept radially outwardsby the surface streamlines, indicated by the directional arrows, suchthat the particle is swept through the inertial trap and deposited inthe reservoir where it is never subject to further reentrainment. Thesame process applies to any debris or loose deposition on the surface ofthe impactor head. However, if a very symmetrical bouncy particle orsubstance makes a near 180° vertical bounce of several milimeters (mm)after hitting a hard, substantially flat, impaction surface, it has achance to be carried by the upper streamlines into the flow of small(less than 15 μm) particles. The present invention eliminates this typeof particle bounce or backscattering.

As more clearly illustrated in FIG. 3, the upper surface 20 of impactorhead 21 is provided with concentric V-shaped grooves 29 having anopening angle indicated at a of about 35°. If the opening angle of thegrooves 29 is less than 90° then an incoming particle in the verticaldirection must scatter forward into a groove. Similarly, opening anglesof less than 45° and 30° will prevent secondary and tertiary backscattering. Due to the grooved surface 20, there is little flat surfaceoriented for large-angle backscatter; and when the grooves 29 are filledwith ambient aerosol, the particle impact tends to be inelastic.Resuspended material which tends not to have a high vertical velocitycomponent will be swept radially outwardly by the surface streamlinesand thus be caught efficiently by the inertial trap 22. In theembodiment illustrated in FIG. 3, the grooves 29 have opening angles aof 35° with a depth indicated at c of 2.0 mm, the downward slopeindicated at b of impactor head surface 20 being 5°. Note that the outerupper periphery of impactor head 21 is curved, as indicated, at 30 toincrease the efficiency of the inertial trap 22. The performance isindependent of groove size and angle, provided that the groove openingangle is about an order of magnitude smaller than the characteristicdiameter of the associated air jet through nozzle 14 so as not to affectthe original flow characteristics.

Various tests have been conducted which establish that there is asignificant reduction in particle bounce when an impactor head isprovided with grooves in accordance with the present invention. Forexample, in tests using symmetrical bouncy particles, the total particlebounce of the grooved impactor was reduced to less than 2% of the bounceof a flat (nongrooved) impactor. For further desciption of the tests,attention is directed to report LBL-11682 entitled "A New InhalableParticulate Impactor with Inertial Trap", bearing a date of January1981, by Billy W. Loo et al, Lawrence Berkeley Laboratory, Berkeley,Calif., said report being incorporated herein by reference.

It has thus been shown that the present invention provides a compactinertial size separator having a sharp particle size cut-off forsampling or monitoring apparatus. Through the use of the inertial trapand grooved impaction surface, traditional problems of particle bounceand reentrainment have been virtually eliminated. An inherent advantageof the inertial trap is that bounce-off or blow-off particles areimmediately removed from the air flow region so that resuspension ofthese materials after prolonged field operation will not be possible.

Since the size separator using inertial trap and grooved impactor ofthis invention is based on the well-understood theory of jet impaction,the various features described above can be generalized readily to othersituations where different flow rates and cutoff points are called for.The apparatus utilizing the inertial trap and grooved impactor isdesigned for quick disassembling for field service and to provide aninhalable particle sampler with a reliable weatherproof inlet. Thus, thepresent invention overcomes the problems of the prior knownparticle-size samplers, thus substantially advancing the state of thisart.

While a particular embodiment of the invention has been illustratedand/or described, modifications will become apparent to those skilled inthe art, and it is intended to cover in the appended claims all suchmodifications as come within the scope of this invention.

I claim:
 1. In an inhalable particulate sampler having at least an inletsection and a particle-size separator section, the improvementcomprising: means for substantially preventing particle bounce andreentrainment of particles over a selected maximum size, comprising aparticle impactor, said impactor being provided with a surface ontowhich substantially all incoming particles are initially directed, saidimpactor surface having a plurality of V-shaped grooves thereon, andmeans which includes said particle impactor for forming an inertial trapadjacent a peripheral area of said particle impactor whereby particlesgreater than a selected maximum size are trapped by said inertial trap.2. The improvement of claim 1, wherein said grooves on the surface ofsaid particle impactor are concentric and have an opening angle of notgreater than 90°.
 3. The improvement of claim 1 or 2, wherein saidsurface of particle impactor has an outwardly and downwardly slope inthe range of about 2° to 8°.
 4. The improvement of claim 3, wherein saidgrooves have an opening angle of about 35° and a depth of about 2.0 mm.5. The improvement of claim 1, wherein said particle impactor consistsof an impactor member having an impactor head on which said groovedsurface is located, said grooves being of a concentric configurationhaving an opening angle or not greater than 90°.
 6. The improvement ofclaim 1, additionally including a reservoir in open communication withsaid inertial trap defining means for collecting at least said particleshaving a size greater than said maximum size, said reservoir beingprovided with a plurality of partitions for preventing spiral excursionsof flow streamlines into said reservoir and the resultant depositing insaid reservoir of undesired particles.
 7. A method for substantiallyeliminating particle bounce and reeintrainment of particles over amaximum size in a particulate-collecting apparatus, comprising the stepsof: providing a particle impactor onto which substantially all incomingparticles are directed, providing the particle impactor with anoutwardly and downwardly sloping upper surface, providing the uppersurface of the particle impactor with a plurality of V-shaped grooveshaving an opening angle not greater than about 90°, and forming aninertial trap about the periphery of the particle impactor, such thatparticles over a maximum size are directed into the inertial trap by thegrooved upper surface of the particle impactor.
 8. The method of claim7, additionally including the step of providing a partitioned reservoirin communication with the inertial trap for collecting the particlespassing through the trap.
 9. The method of claim 7, wherein the V-shapedgrooves are provided by forming same to define a groove widthconstituting approximately an order of magnitude smaller than thecharacteristic diameter of an air jet through an associated nozzledirected onto the grooves.
 10. An improved particle impactor for aninhalable particulate sampler apparatus comprising: a downwardly andoutwardly sloping upper surface onto which substantially all incomingparticles are directed, and a plurality of concentric grooves on saidupper surface, whereby vertical bounce of particles of a selectedmaximum size striking said upper surface of said particle impactor issubstanially reduced.
 11. The improved particle impactor of claim 10,wherein said concentric grooves are of a V-shaped configuration with anopening angle of not greater than 35°.
 12. The improved particleimpactor of claim 11, wherein said upper surface of said impactor has aslope in the range of 2° to 8°.